Origin of the late Paleoproterozoic low-δ18O A-type granites on the southern margin of the North China Craton and their geodynamic mechanism

Abstract A-type granites with 18O-depletion signatures are rather enigmatic as regards not only their typology and petrogenesis but also their rock-forming geodynamic mechanism. Here we report a suite of late Paleoproterozoic low-δ18O A-type granites from the Longwangzhuang pluton on the southern margin of the North China Craton. SIMS zircon U-Pb dating showed that the Longwangzhuang pluton was emplaced at 1613 ± 4 Ma. Combined with previous data, it is clear that the mineral components of Longwangzhuang granites are variable and consist of perthite, quartz, plagioclase, biotite and amphibole (calcic hastingsite, ferroedenite, and sodic ferrogedrite). Geochemically, they are metaluminous to peraluminous and have high alkali contents (K2O+Na2O>8.76 wt.%, K2O/Na2O>1), FeOt/(FeOt+MgO) (>0.95) ratios, Zr+Nb+Ce+Y (>993 ppm) contents, and 10000Ga/Al (>4.31) ratios, resembling A-type granites. In-situ oxygen isotope analysis displayed low zircon δ18O values between 4.0‰ and 4.9‰ with a weighted average of 4.4‰. Based on the Archean two-stage zircon Hf and whole-rock Nd model ages coupled with the low (La/Yb)CN ratios, negative anomalies of Eu and Sr, and high Zr saturation temperatures, and the presence of low pressures (3.3–5.7 kbar) amphiboles, the Longwangzhuang A-type granites were generated by “self-cannibalizing” pre-existing low-δ18O old rocks (i.e., the Taihua Complex) and emplaced at shallow depth. Combined with regional geological observations (i.e., coeval tuffites), a passive rift setting was the most plausible explanation for the generation of the late Paleoproterozoic low δ18O A-type granites on the southern margin of the North China Craton. Such a rifting could likely have lasted at least till 1.6 Ga.